Soya sauce is a salty, brown liquid made from fermented soy beans mixed with some type of roasted grain (wheat, barley, or rice are common), injected with a special yeast mold, and liberally flavored with salt. After being left to age for several months, the mixture is strained and bottled. The sauce's consistency can range from very thin to very thick. Flavors, too, vary by type and have very subtle differences.

Soya sauce is used widely in East and Southeast Asian cuisines and appears in some Western cuisine dishes.

Soy sauce has a peculiar aroma and a meaty taste. It is the chief savory-seasoning agent in Oriental cookery, but it is becoming increasingly popular in many other regions of the world. Once a homemade staple, it has become an important industrial product. Industrialization has altered the production process, changed the raw materials used, standardized the products and modified somewhat their characteristics.

The main ingredients of soy sauce are soybean (or defatted soybean meal), wheat, salt and water. The heart of the manufacturing process is a complex fermentation whereby the carbohydrates are fermented to alcohol and lactic acid and the proteins are broken down to peptides and amino acids. Chemical reactions between the original components and the fermentation products create the color, consistency and aroma of soy sauce. Formulations and processing conditions differ from one country to another and in function of the type of soy sauce to be produced. The process of manufacturing soy sauce can be divided into three stages: preparation of the "Koji", fermentation in brine and product refining.

Soy sauce process flow chart describes the basic outline of the manufacturing process of soy sauce. Soybeans and/or defatted soybean meal are cooked in continuous or batch pressure-cookers and mixed with roasted and coarsely broken wheat. The mass is inoculated with a mould- Aspergillus oryzae (or A. sojae) and incubated in shallow vats or doughnut-shaped circular plate with perforated bottoms and air is forced through the mass. After two or three days of incubation under controlled conditions of temperature and moisture, mould growth covers the entire mass which turns greenish as a result of sporulation. This mass is called "Koji". Koji is the essential ingredient of most fermented products of the Orient. It is a concentrated source of amylolytic and proteolytic enzymes necessary for the decomposition of the carbohydrates and proteins.

The Koji is mixed with brine containing 22 to 25 % salt (weight by volume) and transferred to deep fermentation tanks. Lactic acid bacteria and osmophilic yeast cultures are added and the slurry ( called "Moromi"-mash) is allowed to ferment at controlled temperature and occasional aeration. The high salt concentration effectively inhibits growth of undesirable "wild" microorganisms. The starch is transformed to sugars that are fermented to lactic acid and alcohol. The pH drops from near neutral to 4.7- 4.8. The Moromi is held in the fermentation tanks for 6 to 8 months. In the refining stage, the fermented mash is pressed to separate the sauce from the solid residue. The sauce is filtered, clarified and heated to 70-80o C. Heating is necessary to pasteurize the sauce and to develop the characteristic color and aroma. After final clarification the sauce is bottled. The use of defatted soybean meal instead of whole soybeans is justified, since most of the oil is lost in the residue.

The quality of soy sauce: the higher the nitrogen amino acid level, the better. According to nitrogen content of amino acids soya sauces are ranked as follows: